About 5-10% of the population in the northern half of the United States suffers from winter seasonal affective disorder (SAD). The most typical symptoms include fatigue, increased appetite, weight gain, increased sleep, and depressed mood. SAD patients experience impaired functioning and interpersonal difficulties for about half of each year. High intensity (bright) light treatment is capable of reducing, or even eliminating, the symptoms. However, we do not know whether bright light is better than a placebo, primarily because of the difficulty in finding an appropriate placebo control. Although bright light can help people, it is important to determine whether it has an active, specific antidepressant effect, in addition to the placebo effect, in order to be able to understand the etiology of SAD and develop treatments. We are conducting a study using a unique placebo control treatment that is plausible, inert, and that requires the same behavioral and sleep schedule changes as a light treatment. Preliminary results suggest that there is a small effect of light treatment beyond the placebo effect. However, we need to collect data for a few more winters before we can draw conclusions with any certainty. In this 5-week parallel design study, a baseline week is followed by four treatment weeks. Patients are randomly assigned to morning light (1.5 hrs upon waking), evening light (1.5 hrs before bed) or morning placebo (1.5 hrs upon waking). Patients in all groups adhere to a strict sleep schedule with a fixed, individualized, wake-up time. Interviewer and self-ratings of depression severity are obtained weekly. A subset of patients (those who volunteer) have their core body temperature monitored continuously during the 5-week protocol. These data will help us evaluate the theory that the phase changes in circadian rhythms produced by light are responsible for the antidepressant effect. The temporal course of symptom onset in the fall is studied as another way to elucidate the mechanisms of winter depression.